Recently, a packaging material is mainly constructed from a composite flexible film made of a combination of different polymer materials in view of its strength, protection ability of contents, workability, and marketing effect by printing. The composite film is generally composed of an outer thermoplastic film layer for protecting contents and a sealant thermoplastic film layer. These layers are laminated by a dry lamination wherein a sealant layer is bonded to a laminating film layer through an adhesive applied on the lamination film layer or by an extrusion lamination where a molten plastic film for forming a sealant layer is press-bonded to a laminating film layer optionally having an anchor coat agent applied thereon so as to form a laminate of films. In these methods, a two-part polyurethane adhesive including a main ingredient having an active hydrogen-containing group, such as hydroxyl group, and a curing agent having an isocyanate group has been mainly used in view of its high adhesiveness (for example, Patent Documents 1 and 2).
However, the rate of curing reaction of the two-part polyurethane adhesive is generally not so high. Therefore, to obtain sufficient adhesiveness, the curing must be promoted by a long-term aging as long as for 1 to 5 days after laminating the layers. In addition, if the isocyanate group in the curing agent remains unreacted after curing, the residual isocyanate group is allowed to react with moisture in the ambient atmosphere to generate carbon dioxide which then forms air bubbles in the laminate film.
To solve these problems, a specific polyurethane adhesive for dry laminating is proposed in Patent Document 3, and an epoxy adhesive for laminating is proposed in Patent Document 4.
However, the gas-barrier properties of the polyurethane adhesives proposed in Patent Documents 1 to 3 and the epoxy adhesive proposed in Patent Document 4 are not so high. Therefore, if a packaging material having high gas-barrier properties is intended, an additional gas-barrier layer of various kinds, such as a polyvinylidene chloride (PVDC) coat layer, a polyvinyl alcohol (PVA) coat layer, an ethylene-vinyl alcohol copolymer (EVOH) film layer, a m-xylylene adipamide film layer, and an inorganic film layer of deposited alumina (Al2O3) or silica (Si), must be laminated, increasing the production costs of laminated film and reducing the workability of laminating process.
Since, as compared with other resins, epoxy resins are excellent in many properties, such as adhesiveness to various materials, heat resistance, chemical resistance, electrical properties, and mechanical properties, epoxy resins are finding wide application in various industries, for example, as adhesives for civil engineering and construction. The gas-barrier properties of an epoxy resin composition for use as an adhesive are generally better than those of urethane resin, acrylic resin, and polyolefin resin, but inferior to those of polyvinylidene chloride and polyvinyl alcohol which are known as gas-barrier materials. Therefore, to improve the gas-barrier properties of epoxy resin, various methods are employed, for example, increasing the film thickness, coating the film with other material, and combinedly using filler.
Patent Document 5 proposes a gas-barrier epoxy resin composition which includes an epoxy resin and an amine curing agent. The gas-barrier properties of the proposed epoxy resin composition are good, but further improvement is recently required. In addition, the adhesiveness to polyester is insufficient and its improvement is required.